The present disclosure relates to molded articles that incorporate holograms, more particularly color transmission and reflection holograms. Methods of making and using the same are also disclosed.
Holograms are an increasingly popular mechanism for the authentication of genuine articles, whether it is for security purposes or for brand protection. The use of holograms for these purposes is driven primarily by the relative difficulty with which they can be duplicated. Holograms are created by interfering two coherent beams of light to create an interference pattern and storing that pattern in a holographic recording medium. Information or imagery can be stored in a hologram by imparting the data or image to one of the two coherent beams prior to their interference. The hologram can be read out by illuminating it with beams matching either of the two original beams used to create the hologram and any data or images stored in the hologram will be displayed. As a result of the complex methods required to record holograms, their use for authentication can be seen on articles such as credit cards, software, passports, clothing, and the like.
Two categories of holograms include surface relief structure holograms and volume holograms. Many of the holograms used in security or authentication applications are of the surface relief type, in which the pattern and any data or image contained therein is stored in the structure or deformations imparted to the surface of the recording medium. As a result, the first recorded hologram may be created by the interference of two coherent beams, but duplicates can be created by copying the surface structure using techniques such as embossing. The duplication of holograms is convenient for the mass production of articles such as credit cards or security labels, but it also has the disadvantage that it makes the unauthorized duplication and/or modification of these holograms for use in counterfeit parts possible from the originals using the same mechanism.
Unlike surface holograms, volume holograms are formed in the bulk of a recording medium. Volume holograms have the ability to be multiplexed, storing information at different depths, spatial locations and angles within the bulk recording material and thus have the ability to store greater amount of information, including for example the ability to animate images recorded via spatial, angular or combined angular and spatial multiplexing. In addition, because the holographic pattern is embedded in the material, copying cannot be done using the same techniques as for surface relief holograms, and is in general more difficult to do with high fidelity.
While volume holograms can provide more security against counterfeit duplication than surface relief structure holograms, it would be desirable to increase the security of volume holograms. Increasing the complexity of a volume hologram incorporated into the structure of a product could result in a hologram that would serve as a powerful authenticity tool. Photopolymers have been widely used for volume hologram applications. These photopolymers undergo refractive index changes after optically-induced polymerization following exposure by object and reference light sources. However, photopolymers generally cannot withstand the temperatures and pressures commonly experienced during fabrication processes like co-extrusion or thermal fusion lamination.
Thus, there remains a need for improved holograms for incorporation into the structure of a product such as for security applications.